EPN is an insecticide widely used in agricultural applications. That compound and its use is claimed by Jelinek in U.S. Pat. No. 2,503,390. Jelinek prepared EPN by reacting a thionobenzene phosphonyl dichloride with sodium ethylate to give ethylthionobenzene phosphonyl monochloride which was then reacted with sodium p-nitrophenate to give EPN. The EPN prepared by Jelinek was an oily liquid having a light yellow color and was indicated by Jelinek to be not highly refined.
Shindo et al. in U.S. Pat. No. 3,327,026 describes a process for preparing EPN in which 2-methyl-5-ethylpyridine is used as a catalyst so as to inhibit production of by-products, such as bis-(O-ethyl)-phenylphosphonothioate and bis-(O-4-nitrophenyl)-phenylphosphonothioate.
Technical EPN which is currently available commercially is a light tan to brown composition which typically contains from 85 to 90% EPN, the rest being by-products, side reaction products and impurities. Pure EPN has a melting point of approximately 36.degree. C (e.g. see U.S. Pat. No. 3,896,192, column 7, lines 54-55), and under normal storage or warehouse conditions, particularly in the fall, winter or early spring, technical EPN will solidify in part, or entirely, inside the shipping container due to crystallization of the EPN. This poses the obvious inconvenience that a formulator who wishes to use the technical EPN to prepare an agricultural formulation must melt the material prior to removal from the drum, or must flush it from the drum with hot solvent, or must open the drum in order to remove the active ingredient as a solid or a solid-laden slush. In addition to the inconvenience of such handling, there is a health hazard because of EPN toxicity, particularly if drums are opened and the EPN is removed as a solid or slush.
It is, therefore, desirable to prevent the crystallization of solid EPN in shipping containers or to delay or inhibit such crystallization for a sufficient time period so that the user who receives an EPN shipment will be able to handle the material as a liquid before the onset of undesirable crystallization. True lowering of the EPN crystallization temperature could be achieved by the addition of solvents in accordance with the normal laws of physical chemistry. However, the solvents that have been used in pursuing the problem leading to the present invention were not very effective, and large amounts would be required to keep EPN liquid at, for example, 0.degree. C. It is, therefore, desirable to add to the technical EPN a crystallization inhibitor. As is known to those skilled in the art, a crystallization inhibitor is a material which does not prevent crystallization by virtue of modifying the true crystallization or freezing point or by virtue of acting as a solvent, but rather is a material which interferes with the nucleation of crystals and, once crystals have nucleated, prevents or inhibits or slows down the further growth of crystals in the liquid medium. As is also known in this art, one cannot predict which material would serve as a crystallization inhibitor from equations or known laws.